It is now well known that graphite reinforced composite materials are susceptible to puncture or other severe damage from lightning strikes. This is because the density of the lightning current injected into the graphite fibers at the point of attachment of the lightning arc is high enough to raise the temperature of the graphite fibers well above the vaporization temperature of the resin matrix. This in turn results in explosive pyrolysis of the resin, pressure buildup, and the destruction of several plies of the composite laminate. Moreover, if the laminate is painted, as is ordinarily the case, it has been found that the paint will tend to concentrate the lightning current at a single point, rather than allow it to divide and enter the laminate at several places, and that it will contain the thermal and blast presure effects of the lightning strike, thus enabling them to build up to a greater magnitude. Both of these effects, of course, increase the amount of physical damage to the laminate.
In addition to the physical damage, the higher resistivity of graphite composites, a resistivity approximately five hundred times greater than that of aluminum, for example, results in excessive structural voltage rises during lightning strike flow. Such structural voltage increases may cause damage to the electrical and avionics equipment contained within an airplane, for example, as well as electrical sparking within the fuel tank.
In light of the foregoing, it is clear that it is necessary to minimize the physical damage effects of lightning strikes on graphite composites and to reduce the electrical resistance of such composites. In an attempt to achieve this, a flame of arc-sprayed coating of metal, usually aluminum, of approximately 4 and 6 mils thickness has been applied to the composite, but such flame-sprayed coatings suffer from the disadvantages of being difficult to apply and to maintain and are subject to cracking, which can lead to corrosion and/or blemishes when applied to large surfaces as would be necessary in the case of a structure such as an airplane or other aerospace vehicle.
To provide the required protection for graphite composites, it has also been attempted to add a ply of aluminized glass fabric or a ply of woven wire mesh to the laminate structure. It has been found, however, that unless a symmetrical layup is used, i.e., a ply on the inside as well as on the outside surface of the graphite laminate, aluminized glass cloth may cause warpage of the laminate.
Woven wire mesh has also been found to be difficult to lay up on multiple contoured surfaces. Also, the addition of either the aluminized glass cloth or woven wire mesh would add a significant amount of weight to the laminate structures, which is highly undesirable, especially in the case of an airplane or other aerospace vehicle, where the lighter weight of the graphite composite is an exceptionally desirable attribute and one which contributes to improved fuel consumption.